1. Field of the Invention
The present invention relates to a color image readout apparatus and, more specifically, to a color image readout apparatus which separates a color image into color light components and then reads out thus separated color light components.
2. Description of the Prior Art
Various kinds of color image readout apparatus have been proposed, in which a color image is separated into three primary colors of B, G, and R, and the respective color light components in thus color- separated wavelength ranges are read out by a light receiving device.
For example, there has been proposed, as shown in FIG. 5, a color image readout apparatus comprising a light source 31 for emitting white light including the whole desired wavelength range; an imaging optical system 34 for collecting the light reflected from an original 32 mounted on a platen glass 33; a light receiving device 36 in which three pieces of line sensors 51, 52, and 53, respectively set to have three kinds of spectral sensitivity characteristics for B, G, and R, are arranged in a sub-scanning direction on a single chip; and a beam splitter 37, disposed on an optical path between the imaging optical system 34 and the light receiving device 36, in which dichroic mirrors respectively adapted to reflect only predetermined wavelength ranges of light and a total reflection mirror are laminated together with a transparent glass therebetween. In such a color image readout apparatus, the light emitted from the light source 31 and then reflected by the original 32 is separated by the beam splitter 37 into three color light components of B, G, and R which are in parallel with each other, and then the color light components are respectively received by the line sensors 51, 52, and 53 of the light receiving device 36.
Also, there has been proposed, as shown in FIG. 6, a color image readout apparatus comprising a light source 41 for emitting white light including the whole desired wavelength range; an imaging optical system 44 for collecting the light reflected from an original 42 mounted on a platen glass 43; a light receiving device 46 in which three pieces of line sensors 61, 62, and 63, respectively set to have three kinds of spectral sensitivity characteristics for B, G, and R, are arranged in a sub-scanning direction on a single chip; and first and second beam splitters 47 and 48, disposed on an optical path between the imaging optical system 44 and the light receiving device 46, in each of which dichroic mirrors respectively adapted to reflect only predetermined wavelength ranges of light and a total reflection mirror are laminated together with a transparent glass therebetween. In such a color image readout apparatus, the light emitted from the light source 41 and then reflected by the original 42 is separated by the first beam splitter 47 into three color light components of B, G, and R which are in parallel with each other, and then the color light components are reflected by the second beam splitter 48 so as to be respectively received by the line sensors 61, 62, and 63 of the light receiving device 46. Such a case where two pieces of the beam splitters 47 and 48 are used is advantageous in that the optical path lengths of the respective color light components can be made substantially equal to each other.
Though a halogen lamp is often used as the light source in such a color image readout apparatus, it has a characteristic that its spectral energy has a smaller short wavelength component and a greater long wavelength component in the visible light range. Consequently, when the halogen lamp is employed in a color image readout apparatus such as those mentioned above, the readout signals of the respective colors may become out of balance. Accordingly, amplification factors for obtaining the respective readout signals should be adjusted so as to attain substantially the same output level. When the amplification factors differ among the readout signals, however, S/N ratios may vary among the respective colors. As a result, upon final image processing such as masking, the S/N ratio of the signal concerning the B light component, which has a short wavelength, may become low, thereby greatly deteriorating its image quality.
Accordingly, there has been proposed a color image readout apparatus Japanese Unexamined Patent Publication No. 7-221930) in which, in place of the transparent glass constituting the beam splitter, an optical filter such as ND filter or color compensating filter is used such that the color light components forming images on the respective line sensors have substantially the same light quantity, thereby attaining substantially the same amplification factor for the signals corresponding to the respective color light components so as to keep a favorable balance between their S/N ratios, allowing the readout operation to be performed with a high accuracy.
In a beam splitter such as that mentioned above in which a dichroic mirror and a total reflection mirror are laminated together, however, it is very difficult to manufacture a dichroic mirror having 100% transmittance and reflectance for respective color light components. Consequently, in the vicinity of the boundary wavelength between two color light components to be separated, the color light component to be reflected may be transmitted, and thus transmitted color light component may be reflected by the dichroic mirror so as to be made incident on a line sensor which should receive another color light component. Namely, as shown in FIG. 5, of the white light incident on the beam splitter 37, the B light component is reflected by a first dichroic mirror 37A, while the G and R light components are transmitted therethrough. Of thus transmitted G light component, a wavelength component near the wavelength range of B light component may contain a part of the B light component. Such a part of the B light component may be reflected by a second dichroic mirror 37B so as to be made incident on the line sensor 52 for receiving the G light component. Similarly, the R light component transmitted through the second dichroic mirror 37B may contain a part of the G light component and a small amount of B light component, which may be made incident on the line sensor for receiving the R light component.
Also, of the G light component, the most part of a portion in the vicinity of the wavelength range of the B light component is reflected by the second dichroic mirror 37B, and then a part thereof is reflected by the inner surface of the dichroic mirror 37A and further by the second dichroic mirror 37B. Consequently, multiple reflection may occur within the beam splitter 37, and a multiple reflection light component L10 (indicated by a broken line) may be made incident on the line sensor 53 for receiving the R light component in the light receiving device 36. Also, in the color image readout apparatus shown in FIG. 6, multiple reflection light components L11 and L12 may be generated as indicated by broken lines therein. In the reflectance characteristics of B, G, and R shown in FIG. 7, such a multiple reflection light component is generated at a hatched area of the wavelength range.
Thus, an unnecessary multiple reflection light component generated between dichroic mirrors constituting a beam splitter may be made incident on a line sensor which is different from the one that should receive it, whereby color contamination may occur in the line sensor, thus deteriorating the color reproducibility of the resulting image. Also, when an error occurs in the gap amount or parallelism of the dichroic mirrors constituting the beam splitter, a positional deviation may be generated between the normal and contaminated color light components incident on the beam splitter, thereby forming a ghost in the resulting image. Further, depending on the optical path of the contaminated color light component, the normal and contaminated color light components may have different optical path lengths, so as to form images on the beam splitter in a defocused state, thereby generating a flare in the resulting image.